High-Hardness Steel Product and Method of Manufacturing the Same

ABSTRACT

Described is a hot-rolled steel strip product including a composition consisting of, in terms of weight percentages, 0.14% to 0.35% C, 0% to 0.5% Si, 0.05% to 0.40% Mn, 0.1% or less Al, 0.1% to 0.4% Cu, 0.2% to 0.9% Ni, 0.2% to 0.9% Cr, 0.2% or less Mo, 0.005% or less Nb, 0.035% or less Ti, 0.05% or less V, 0.0005% to 0.0050% B, 0.025% or less P, 0.008% or less S, 0.01% or less N, 0.01% or less Ca, and the remainder being Fe and inevitable impurities, wherein the steel product has a Brinell hardness in the range of 420 to 580 HBW.

FIELD OF INVENTION

The present invention relates to a high-hardness steel strip productexhibiting a good balance of high hardness and excellent mechanicalproperties such as impact strength and formability/bendability. Thepresent invention further relates to a method of manufacturing thehigh-hardness steel strip product.

BACKGROUND

High hardness has a direct effect on wear resistance of a steel product,the higher hardness the better wear resistance. By high hardness it ismeant that the Brinell hardness is at least 450 HBW and especially inthe range of 500 HBW to 650 HBW.

Wear resistant steels are also known as abrasion resistant steels. Theyare used in applications in which high resistance against abrasive andshock wear is required. Such applications can be found in e.g. miningand earth moving industry, and waste transportation. Wear resistantsteels are used for instance in gravel truck's bodies and excavatorbuckets, whereby longer service time of the vehicle components isachieved due to the high hardness provided by the wear resistant steels.

Wear resistant steels can also function as structural steels for makingconstruction components if the wear resistant steels have sufficientmechanical properties such as formability, weldability and fatigueresistance that comply with national standards. The advantage of usingwear resistant steels in the structural part for construction purposesis that less welding is needed and the weight can be lowered.

Such high hardness in a steel product is typically obtained bymartensitic microstructure produced by quench hardening steel alloyhaving high content of carbon (0.41-0.50 wt. %) after austenitization inthe furnace. In this process steel plates are first hot-rolled, slowlycooled to room temperature from the hot-rolling heat, reheated toaustenitization temperature, equalized and finally quench hardened. Thisprocess is hereinafter referred to as the reheating and quenching (RHQ)process. Examples of steels produced in this way are wear resistantsteels disclosed in CN102199737 or some commercial wear resistantsteels. Due to the relatively high content of carbon, which is requiredto achieve the desired hardness, the resulting martensite reactioncauses significant internal residual stresses to the steel. This isbecause the higher the carbon content the higher the lattice distortion.Therefore, this type of steel is very brittle and can even crack duringthe quench hardening. Due to the high carbon content these steels havedeteriorated impact strength, poor formability or bendability, and lowresistance to stress corrosion cracking (SCC). Stress corrosion crackingis the cracking induced from the combined influence of tensile stressand a corrosive environment. To overcome these drawbacks, a temperingstep after quench hardening can be introduced to improve mechanicalproperties. This however increases the processing efforts and costs.

CN102392186 and CN103820717 relate to RHQ steel plates having relativelylow carbon content (0.25-0.30 wt. % in CN102392186; 0.22-0.29 wt. % inCN103820717) and also relatively low manganese content. A tempering stepafter quench hardening is required for making such RHQ steel plates,which inevitably increases the processing efforts and costs.

EP2695960 relates to an abrasion-resistant steel product exhibitingexcellent resistance to stress corrosion cracking, which steel sheet canbe made in a process where direct quenching (DQ) may be performedimmediately after hot rolling, without the reheating treatment after hotrolling as in the RHQ process. The steel sheet of EP2695960 has arelatively low carbon content (0.20-0.30 wt. %) and a relatively highmanganese content (0.40-1.20 wt. %). In order to increase the resistanceto stress corrosion cracking, the base phase or main phase of themicrostructure of the steel product of EP2695960 must be made oftempered martensite. On the other hand, the area fraction of untemperedmartensite is restricted to 10% or less because the resistance to stresscorrosion cracking is reduced in the presence of untempered martensite.In balancing abrasion resistance and resistance to stress corrosioncracking, the steel product of EP2695960 has a surface hardness of 520HBW or less.

SUMMARY OF INVENTION

The present invention extends the utilization of the cost-effectivethermomechanically controlled processing (TMCP) in conjunction withdirect quenching (DQ) and possibly also tempering to produce ahigh-hardness steel strip product exhibiting excellentformability/bendability and impact strength values.

In view of the state of art, the object of the present invention is tosolve the problem of providing a high-hardness steel strip productexhibiting excellent formability/bendability and impact strength values.The problem is solved by the combination of specific alloy designs withcost-efficient TMCP procedures which produces a metallographicmicrostructure comprising mainly martensite.

In a first aspect, the present invention provides a hot-rolled steelstrip product comprising a composition consisting of, in terms of weightpercentages (wt. %):

C 0.14-0.35, preferably 0.17-0.31, more preferably 0.20-0.28 Si 0-0.5,preferably 0.01-0.50, more preferably 0.03-0.25 Mn 0.05-0.40, preferably0.05-0.30 Al 0-0.1, preferably 0-0.08 Cu 0.1-0.4, preferably 0.10-0.35Ni 0.2-0.9, preferably 0.3-0.8, more preferably 0.3-0.7 Cr 0.2-0.9,preferably 0.3-0.8, more preferably 0.3-0.7 Mo 0-0.2, preferably 0-0.1Nb 0-0.005 Ti 0-0.035 V 0-0.05 B 0.0005-0.0050, preferably 0.0008-0.0040P 0-0.025, preferably 0-0.020 S 0-0.008, preferably 0-0.005 N 0-0.01,preferably 0-0.005 Ca 0-0.01, preferably 0-0.005, more preferably0-0.003

remainder Fe and inevitable impurities.

The steel product has a low content of Mn, which is important forimproving impact toughness and bendability.

The levels of Cr and Ni are set to improve hardenability. The level ofNi is further set to improve impact toughness and formability.

The level of Nb should be restricted to the lowest possible to increaseformability or bendability of the steel product. Elements such as Nb maybe present as residual contents that are not purposefully added.

The difference between residual contents and unavoidable impurities isthat residual contents are controlled quantities of alloying elements,which are not considered to be impurities. A residual content asnormally controlled by an industrial process does not have an essentialeffect upon the alloy.

In a second aspect, the present invention provides a method formanufacturing hot-rolled steel strip product comprising the followingsteps of

-   -   providing a steel slab consisting of, in terms of weight        percentages (wt. %):

C 0.14-0.35, preferably 0.17-0.31, more preferably 0.20-0.28 Si 0-0.5,preferably 0.01-0.50, more preferably 0.03-0.25 Mn 0.05-0.40, preferably0.05-0.30 Al 0-0.1, preferably 0-0.08 Cu 0.1-0.4, preferably 0.10-0.35Ni 0.2-0.9, preferably 0.3-0.8, more preferably 0.3-0.7 Cr 0.2-0.9,preferably 0.3-0.8, more preferably 0.3-0.7 Mo 0-0.2, preferably 0-0.1Nb 0-0.005 Ti 0-0.035 V 0-0.05 B 0.0005-0.0050, preferably 0.0008-0.0040P 0-0.025, preferably 0-0.020 S 0-0.008, preferably 0-0.005 N 0-0.01,preferably 0-0.005 Ca 0-0.01, preferably 0-0.005, more preferably0-0.003

-   -   remainder Fe and inevitable impurities;    -   heating the steel slab to the austenitizing temperature of        1150-1300° C.;    -   hot-rolling to the desired thickness at a temperature in the        range of Ar₃ to 1250° C., wherein the finish rolling temperature        is in the range of 800° C. to 960° C., preferably 870° C.-940°        C., more preferably 880° C.-930° C.; and    -   direct quenching the hot-rolled steel strip product to a cooling        end and coiling temperature of 450° C. or less, preferably        250° C. or less, more preferably 150° C. or less, and even more        preferably 100° C. or less.

Optionally, a step of temper annealing is performed on the directquenched product at a temperature in the range of 150° C.-250° C.However, the step of temper annealing is not required according to thepresent invention.

The steel product is a steel strip having a thickness of 10 mm or less,preferably 8 mm or less.

The obtained steel product has a microstructure comprising, in terms ofvolume percentages (vol. %), at least 90 vol. % martensite, preferablyat least 95 vol. % martensite, and more preferably at least 98 vol. %martensite, measured from ¼ thickness of the steel strip product. Themartensitic structure may be untempered, autotempered and/or tempered.Typically, the microstructure also comprises retained austenite,bainite, ferrite and/or cementite.

The obtained steel product has a prior austenite grain size of 50 μm orless, preferably 30 μm or less, more preferably 20 μm or less, measuredfrom ¼ thickness of the steel strip product.

The aspect ratio of a prior austenite grain structure is one of thefactors affecting a steel product's impact toughness and bendability. Inorder to improve impact toughness, the prior austenite grain structureshould have an aspect ratio of at least 1.5, preferably at least 2, andmore preferably at least 3. In order to improve bendability, the prioraustenite grain structure should have an aspect ratio of 7 or less,preferably 5 or less, and more preferably 1.5 or less. The obtainedsteel product according to the present invention has a prior austenitegrain structure with an aspect ratio in the range of 1.5-7, preferably1.5-5, and more preferably 2-5, which ensures that a good balance ofexcellent impact toughness and excellent bendability can be achieved.

The steel product has a good balance of high hardness and excellentmechanical properties such as impact strength andformability/bendability.

The steel product has at least one of the following mechanicalproperties: a Brinell hardness in the range of 420-580 HBW, preferably450-550 HBW, more preferably 460-530 HBW, and even more preferably470-530 HBW; a Charpy-V impact toughness of at least 50 J/cm² at atemperature of −40° C.

The steel product exhibits excellent bendability or formability. Thesteel product has a minimum bending radius of 3.2 t or less in ameasurement direction longitudinal to the rolling direction wherein thebending axis is longitudinal to rolling direction; a minimum bendingradius of 2.5 t or less in a measurement direction transversal to therolling direction wherein the bending axis is transversal to rollingdirection; and wherein t is the thickness of the steel strip product.

DETAILED DESCRIPTION OF THE INVENTION

The term “steel” is defined as an iron alloy containing carbon (C).

The term “Brinell hardness (HBW)” is a designation of hardness of steel.The Brinell hardness test is performed by pressing a spherical tungstencarbide ball with a diameter of 10 mm against a clean prepared surfaceof a metal sheet using a 3000 kilogram force, producing an impression,measured and given a special numerical value. A spherical tungstencarbide ball with a diameter of 5 mm and a load of 750 kilogram forceare applied to test samples with thinner gauges, e.g. 3 mm in thickness.

The term “gauge” refers generally to a measure of the thickness of ametal sheet.

The term “ultimate tensile strength (UTS, R_(m))” refers to the limit,at which the steel fractures under tension, thus the maximum tensilestress.

The term “yield strength (YS, R_(p)0.2)” refers to 0.2% offset yieldstrength defined as the amount of stress that will result in a plasticstrain of 0.2%.

The term “total elongation (TEL)” refers to the percentage by which thematerial can be stretched before it breaks; a rough indicator offormability, usually expressed as a percentage over a fixed gauge lengthof the measuring extensometer. Two common gauge lengths are 50 mm (A50)and 80 mm (A80).

The term “minimum bending radius (R_(i))” is used to refer to theminimum radius of bending that can be applied to a test sheet withoutoccurrence of cracks.

The term “bendability” refers to the ratio of R_(i) and the sheetthickness (t). The term “bendability” can also be used interchangeablywith “formability” in the context of the current description.

The term “heat-affected zone (HAZ)” refers to a non-melted area of ametal material that has experienced changes in its material propertiesas a result of exposure to high temperatures. The alterations inmaterial properties are usually a result of welding or high-heat cuttingprocedures. The HAZ is identified as the area between the weld or cutand the base metal material. These areas can vary in size and severitydepending on the properties of the materials involved, the intensity andconcentration of heat, and the process employed.

The alloying content of steel together with the processing parametersdetermines the microstructure which in turn determines the mechanicalproperties of the steel.

Alloy design is one of the first issues to be considered when developinga steel product with targeted mechanical properties. Next the chemicalcomposition according to the present invention is described in moredetails, wherein % of each component refers to weight percentage.

Carbon C is used in the range of 0.14% to 0.35%.

C alloying increases strength of steel by solid solution strengthening,and hence C content determines the strength level. C is used in therange of 0.14% to 0.35% depending on targeted hardness. If the carboncontent is less than 0.14%, it is difficult to achieve a Brinellhardness of more than 420 HBW. C is also an austenite stabilizingelement. However, C has detrimental effects on weldability, impacttoughness, formability or bendability, and resistance to stresscorrosion cracking. Therefore, C content is set to not more than 0.35%.

Preferably, C is used in the range of 0.17% to 0.31%, and morepreferably 0.20% to 0.28%.

Silicon Si is used in an amount of 0.5% or less.

Si is added to the composition to facilitate formation of a protectiveoxide layer under corrosive climate conditions, which provides goodresistance against climatic corrosion and increases the durability of apaint layer that is easily damaged or removed from machines surfaces dueto wear. Si is effective as a deoxidizing or killing agent that canremove oxygen from the melt during a steelmaking process. Si alloyingenhances strength by solid solution strengthening, and enhances hardnessby increasing austenite hardenability. Also the presence of Si canstabilize retained austenite. However, silicon content of higher than0.5% may unnecessarily increase carbon equivalent (CE) value therebyweakening the weldability. Furthermore, surface quality may bedeteriorated if the Si level is excessively high.

Preferably, Si is used in the range of 0.01% to 0.50%, and morepreferably 0.03% to 0.25%.

Manganese Mn is used in the range of 0.05% to 0.40%.

Mn alloying lowers martensite start temperature (M_(s)) and martensitefinish temperature (M_(f)), which can suppress autotempering ofmartensite during quenching. Reduced autotempering of martensite leadsto higher internal stresses that may enhance the risk for quench-inducedcracking or distortion of shape. Although a lower degree of autotemperedmartensitic microstructures is beneficial to higher hardness, itsnegative effects on impact strength should not be underestimated.

Mn alloying enhances strength by solid solution strengthening, andenhances hardness by increasing austenite hardenability. However, if theMn content is too high, hardenability of the steel will increase at theexpense of impact toughness. Excessive Mn alloying may also lead to C—Mnsegregation and formation of MnS, which could induce formation ofinitiation sites for pitting corrosion and stress corrosion cracking.

Thus, Mn is used in an amount of at least 0.05% to ensure hardenability,but not more than 0.40% to avoid the harmful effects as described aboveand to ensure excellent mechanical properties such as impact strengthand bendability. Preferably, a low level of Mn is used in the range of0.05% to 0.30% to further improve the bendability.

Aluminum Al is used in the range of 0.1% or less.

Al is effective as a deoxidizing or killing agent that can remove oxygenfrom the melt during a steelmaking process. Al removes N by formingstable AlN particles and provides grain refinement, which is beneficialto high toughness. Also, Al stabilizes retained austenite. However, anexcess of Al may increase non-metallic inclusions thereby deterioratingcleanliness.

Preferably, Al is used in the range of 0.08% or less.Copper Cu is used in the range of 0.1% to 0.4%.

Cu is added to the composition to facilitate formation of a protectiveoxide layer under corrosive climate conditions, which provides goodresistance against climatic corrosion and increases the durability of apaint layer that is easily damaged or removed from machines surfaces dueto wear. Cu may promote formation of low carbon bainitic structures,cause solid solution strengthening and contribute to precipitationstrengthening. Cu may also have beneficial effects of inhibiting stresscorrosion cracking. When added in excessive amounts, Cu deterioratesfield weldability and the heat affected zone (HAZ) toughness. Therefore,the upper limit of Cu is set to 0.4%.

Preferably, Cu is used in the range of 0.10% to 0.35%.

Nickel Ni is used in the range of 0.2% to 0.9%.

Ni is used to avoid quench induced cracking and also to improvetoughness and formability. Ni is an alloying element that improvesaustenite hardenability thereby increasing strength with no or marginalloss of impact toughness and/or heat-affected zone (HAZ) toughness. Nialso improves surface quality thereby preventing pitting corrosion, i.e.initiation site for stress corrosion cracking. Ni is added to thecomposition to facilitate formation of a protective oxide layer undercorrosive climate conditions, which provides good resistance againstclimatic corrosion and increases the durability of a paint layer that iseasily damaged or removed from machines surfaces due to wear. However,nickel contents of above 0.9% would increase alloying costs too muchwithout significant technical improvement. An excess of Ni may producehigh viscosity iron oxide scales which deteriorate surface quality ofthe steel product. Higher Ni contents also have negative impacts onweldability due to increased CE value and cracking sensitivitycoefficient.

Ni is preferably used in the range of 0.3% to 0.8%, and more preferably0.3% to 0.7%.

Chromium Cr is used in the range of 0.2% to 0.9%.

Cr is added to the composition to facilitate formation of a protectiveoxide layer under corrosive climate conditions, which provides goodresistance against climatic corrosion and increases the durability of apaint layer that is easily damaged or removed from machines surfaces dueto wear. Cr alloying provides better resistance against pittingcorrosion thereby preventing stress corrosion cracking at an earlystage. As mid-strength carbide forming element Cr increases the strengthof both the base steel and weld with marginal expense of impacttoughness. Cr alloying also enhances strength and hardness by increasingaustenite hardenability. However, if Cr is used in an amount above 0.9%the heat-affected zone (HAZ) toughness as well as field weldability maybe adversely affected.

Preferably, Cr is used in the range of 0.3% to 0.8%, and more preferably0.3% to 0.7%.

Molybdenum Mo is used in the range of 0.2% or less.

Mo alloying improves impact strength, low-temperature toughness andtempering resistance. The presence of Mo enhances strength and hardnessby increasing austenite hardenability. Mo can be added to thecomposition to provide hardenability in place of Mn. In the case of Balloying, Mo is usually required to ensure the effectiveness of B.However, Mo is not an economically acceptable alloying element. If Mo isused in an amount of above 0.2% toughness may be deteriorated therebyincreasing the risk of brittleness. An excessive amount of Mo may alsoreduce the effect of B. Furthermore, the inventors have noticed that Moalloying retards recrystallization of austenite thereby increasing theaspect ratio of a prior austenite grain structure. Therefore, the levelof Mo content should be carefully controlled to prevent excessiveelongation of the prior austenite grains which may deterioratebendability of the steel product.

Preferably, Mo is used in the range of 0.1% or less.

Niobium Nb is used in an amount of 0.005% or less.

Nb forms carbides NbC and carbonitrides Nb(C,N). Nb is considered to bethe major grain refining element. Nb contributes to strengthening andtoughening of steels. Yet, Nb addition should be limited to 0.005% sincean excess of Nb deteriorates bendability, in particular when directquenching is applied and/or when Mo is present in the composition.Furthermore, Nb can be harmful for heat-affected zone (HAZ) toughnesssince Nb may promote the formation of coarse upper bainite structure byforming relatively unstable TiNbN or TiNb(C,N) precipitates. The levelof Nb should be restricted to the lowest possible to increaseformability or bendability of the steel product.

Titanium Ti is used in an amount of 0.035% or less.

TiC precipitates are able to deeply trap a significant amount ofhydrogen H, which decreases the H diffusivity in the materials andremoves some of the detrimental H from the microstructure to preventstress corrosion cracking. Ti is also added to bind free N that isharmful to toughness by forming stable TiN that together with NbC canefficiently prevent austenite grain growth in the reheating stage athigh temperatures. TiN precipitates can further prevent grain coarseningin the heat-affected zone (HAZ) during welding thereby improvingtoughness. TiN formation suppresses BN precipitation, thereby leaving Bfree to make its contribution to hardenability. However, if Ti contentis too high, coarsening of TiN and precipitation hardening due to TiCdevelop and toughness may be deteriorated. Therefore, it is necessary torestrict Ti so that it does not exceed 0.035%.

Vanadium V is used in an amount of 0.05% or less.

V has substantially the same but smaller effects as Nb. V4C3precipitates are able to deeply trap a significant amount of hydrogen H,which decreases the H diffusivity in the materials and removes some ofthe detrimental H from the microstructure to prevent hydrogen inducedcracking (HIC). V is a strong carbide and nitride former, but V(C,N) canalso form and its solubility in austenite is higher than that of Nb orTi. Thus, V alloying has potential for dispersion and precipitationstrengthening, because large quantities of V are dissolved and availablefor precipitation in ferrite. However, an addition of more than 0.05% Vhas negative effects on weldability, hardenability and alloying cost.

Boron B is used in the range of 0.0005% to 0.0050%.

B is a well-established microalloying element to increase hardenability.Boron can be added to retard phosphorus segregation to grain boundariesthereby reducing embrittlement during welding in the heat-affected zone(HAZ). Effective B alloying requires the presence of Ti to preventformation of BN. In the presence of B, Ti content can be lowered to beless than 0.02%, which is beneficial for toughness. However,hardenability deteriorates if the B content exceeds 0.005%.

Preferably, B is used in the range of 0.0008% to 0.0040%.

Calcium Ca is used in an amount of 0.01% or less.

Ca addition during a steelmaking process is for refining, deoxidation,desulphurization, and control of shape, size and distribution of oxideand sulphide inclusions. Ca is usually added to improve subsequentcoating. However, an excessive amount of Ca should be avoided to achieveclean steel thereby preventing the formation of calcium sulfide (CaS) orcalcium oxide (CaO) or mixture of these (CaOS) that may deteriorate themechanical properties such as bendability and stress corrosion cracking(SCC) resistance.

Preferably, Ca is used in an amount of 0.005% or less, and morepreferably 0.003% or less to ensure excellent mechanical properties suchas impact strength and bendability.

Unavoidable impurities can be phosphor P, sulfur S and nitrogen N. Theircontent in terms of weight percentages (wt. %) is preferably defined asfollows:

P 0-0.025, preferably 0-0.020 S 0-0.008, preferably 0-0.005 N 0-0.01,preferably 0-0.005

Other inevitable impurities may be hydrogen H, oxygen O and rare earthmetals (REM) or the like. Their contents are limited in order to ensureexcellent mechanical properties, such as impact toughness.

Austenite to martensite transformation in steels depends largely on thefollowing factors: chemical composition and some processing parameters,mainly reheating temperature, cooling rate and cooling temperature. Withregard to chemical composition, some alloying elements have a greaterimpact than others while others have a negligible impact. Equationsdescribing austenite hardenability may be used to assess the impact ofdifferent alloying elements on martensite formation during cooling. Onesuch equation is presented below. From this equation we can see thatcarbon has the biggest impact, Mn, Mo and Cr have an intermediate impactwhile Si and Ni have a lesser impact.

Furthermore, the equation shows that any single element is not crucialfor martensite formation and that the absence of one element may becompensated with the amount of other alloying elements and processingparameters, such as e.g. cooling rate.

$D_{i} = {6 \times {\exp\left\lbrack {7.1 \times \left( {C + \frac{Mn}{5.87} + \frac{Mo}{3.13} + \frac{Cr}{6.28} + \frac{Si}{18} + \frac{Ni}{15}} \right)} \right\rbrack}}$

The steel product with the targeted mechanical properties is produced ina process that determines a specific microstructure which in turndictates the mechanical properties of the steel product.

The first step is to provide a steel slab by means of, for instance aprocess of continuous casting, also known as strand casting.

In the reheating stage, the steel slab is heated to the austenitizingtemperature of 1150-1300° C., and thereafter subjected to a temperatureequalizing step that may take 30 to 150 minutes. The reheating andequalizing steps are important for controlling the austenite graingrowth. An increase in the heating temperature can cause dissolution andcoarsening of alloy precipitates, which may result in abnormal graingrowth.

The final steel product has a prior austenite grain size of 50 μm orless, preferably 30 μm or less, more preferably 20 μm or less, measuredfrom ¼ thickness of the steel strip product.

In the hot rolling stage the slab is hot rolled to the desired thicknessat a temperature in the range of Ar₃ to 1250° C., wherein the finishrolling temperature (FRT) is in the range of 800° C. to 960° C.,preferably 870° C.-940° C., more preferably 880° C.-930° C.

The aspect ratio of a prior austenite grain structure is one of thefactors affecting a steel product's impact toughness and bendability. Inorder to improve impact toughness, the prior austenite grain structureshould have an aspect ratio of at least 1.5, preferably at least 2, andmore preferably at least 3. In order to improve bendability, the prioraustenite grain structure should have an aspect ratio of 7 or less,preferably 5 or less, and more preferably 1.5 or less. A desired aspectratio of prior austenite grains can be achieved by adjusting a number ofparameters such as finish rolling temperature, strain/deformation,strain rate, and/or alloying with the elements such as Mo that retardrecrystallization of austenite.

The obtained steel product according to the present invention has aprior austenite grain structure with an aspect ratio in the range of1.5-7, preferably 1.5-5, and more preferably 2-5, which ensures that agood balance of excellent impact toughness and excellent bendability canbe achieved.

The obtained steel strip product has a thickness of 10 mm or less,preferably 8 mm or less.

The hot-rolled steel strip product is direct quenched to a cooling endand coiling temperature of 450° C. or less, preferably 250° C. or less,more preferably 150° C. or less, and even more preferably 100° C. orless. The cooling rate is at least 30° C./s.

The direct quenched steel strip product is coiled at temperature of 450°C. or less, preferably 250° C. or less, more preferably 150° C. or less,and even more preferably 100° C. or less.

The obtained steel strip product has a microstructure comprising, interms of volume percentages (vol. %), at least 90 vol. % martensite,preferably at least 95 vol. % martensite, and more preferably at least98 vol. % martensite, measured from ¼ thickness of the steel stripproduct. The martensitic structure may be untempered, autotemperedand/or tempered. Preferably, the microstructure comprises 1 vol. % orless retained austenite, and more preferably 0.5 vol. % or less retainedaustenite. Typically, the microstructure also comprises bainite,ferrite, pearlite and/or cementite.

Optionally, an extra step of temper annealing is performed at atemperature in the range of 150° C.-250° C.

The steel strip product has a good balance of hardness and othermechanical properties such as excellent impact strength and excellentformability/bendability.

The steel strip product has a high Brinell hardness in the range of420-580 HBW, preferably 450-550 HBW, more preferably 460-530 HBW, andeven more preferably 470-530 HBW.

The steel strip product with high hardness has a Charpy-V impacttoughness of at least 50 J/cm² at a temperature of −40° C. therebyfulfilling the conventional impact strength requirements.

The steel strip product exhibits excellent bendability or formability.The steel product has a minimum bending radius (R_(i)) of 3.2 t or lessin a measurement direction longitudinal to the rolling direction whereinthe bending axis is longitudinal to rolling direction; a minimum bendingradius (R_(i)) of 2.5 t or less in a measurement direction transversalto the rolling direction wherein the bending axis is transversal torolling direction; and wherein t is the thickness of the steel stripproduct.

The following examples further describe and demonstrate embodimentswithin the scope of the present invention. The examples are given solelyfor the purpose of illustration and are not to be construed aslimitations of the present invention, as many variations thereof arepossible without departing from the scope of the invention.

The chemical compositions used for producing the tested steel stripproducts are presented in Table 1. Steel types A-C are the inventivecompositions according to the present disclosure. Steel types D and Eare comparative compositions which comprise a relatively high Mn contentof 1.20 wt. % and 1.19 wt. % respectively (Table 1).

The manufacturing conditions for producing the tested steel stripproducts are presented in Table 2.

The mechanical properties of the tested steel strip products arepresented in Table 3.

Microstructure

Microstructure can be characterized from SEM micrographs and the volumefraction can be determined using point counting or image analysismethod. The microstructures of the tested inventive examples no. 1-3 allhave a main phase of martensite in an amount of at least 90 vol. %.

Brinell Hardness HBW

The Brinell hardness test is performed by pressing a spherical tungstencarbide ball with a diameter of 10 mm against a clean prepared surfaceof the steel strip samples with a thickness of 6 mm using a 3000kilogram force, producing an impression, measured and given a specialnumerical value. For the strip samples with a thickness of 3 mm, aspherical tungsten carbide ball with a diameter of 5 mm and a load of750 kilogram force are applied. The measurement is done perpendicular tothe upper surface of the steel sheet at 10-15% depth from the steelsurface. As shown in Table 3, each one of the inventive examples no. 1-3exhibits a Brinell harness in the range of 467-489 HBW. The comparativeexamples no. 4 exhibits a Brinell harness of 485 HBW while thecomparative example no. 5 exhibits a Brinell harness of 502 HBW.

Charpy-V Impact Toughness

The impact toughness values at −40° C. are obtained by Charpy V-notchtests according to the ISO 148 standard. Each one of the inventiveexamples no. 1-3 has a Charpy-V impact toughness in the range of 78-118J/cm² at a temperature of −40° C. if the measurement direction islongitudinal to the rolling direction. Each one of the inventiveexamples no. 1-3 has a Charpy-V impact toughness in the range of 65-90J/cm² at a temperature of −40° C. if the measurement direction istransversal to the rolling direction. The impact toughness of theinventive examples no. 1-3 is improved compared to the comparativeexamples no. 4 and 5.

Elongation

Elongation was determined according ISO 6892 standard using longitudinalspecimens. The mean value of total elongation (A80) of the inventiveexamples no. 1, 2 and 3 is 4.5, 7.6 and 7.7 respectively (Table 3).Thecomparative examples no. 4 and 5 have better elongation values than theinventive examples no. 1-3 at the expense of Charpy-V impact toughnessand bendability.

Bendability

The bend test consists of subjecting a test piece to plastic deformationby three-point bending, with one single stroke, until a specified angle90° of the bend is reached after unloading. The inspection andassessment of the bends is a continuous process during the whole testseries. This is to be able to decide if the punch radius (R) should beincreased, maintained or decreased. The limit of bendability (R/t) for amaterial can be identified in a test series if a minimum of 3 m bendinglength, without any defects, is fulfilled with the same punch radius (R)both longitudinally and transversally. Cracks, surface necking marks andflat bends (significant necking) are registered as defects.

According to the bend tests, each one of the inventive examples no. 1-3has a minimum bending radius (R_(i)) of 2.8 t or less in a measurementdirection longitudinal to the rolling direction; a minimum bendingradius (R_(i)) of 2.0 t or less in a measurement direction transversalto the rolling direction; and wherein t is the thickness of the steelstrip product (Table 3). The comparative examples no. 4 and 5 exhibit aminimum bending radius (R_(i)) of 3.7 t and 3.3 t respectively in ameasurement direction longitudinal to the rolling direction, and aminimum bending radius (R_(i)) of 3.0 t and 2.7 t respectively in ameasurement direction transversal to the rolling direction (Table 3).

Yield Strength

Yield strength was determined according ISO 6892 standard usinglongitudinal specimens. Each one of the inventive examples no. 1-3 has amean value of yield strength (R_(p)0.2) in the range of 1310 MPa to 1413MPa measured in the longitudinal direction (Table 3). The comparativeexamples no. 4 and 5 have a mean value of yield strength (R_(p)0.2) of1375 MPa and 1397 MPa respectively, measured in the longitudinaldirection (Table 3).

Tensile Strength

Ultimate tensile strength (R_(m)) was determined according ISO 6892standard using longitudinal specimens. Each one of the inventiveexamples no. 1-3 has a mean value of ultimate tensile strength (R_(m))in the range of 1511 MPa to 1609 MPa, measured in the longitudinaldirection (Table 3). The comparative examples no. 4 and 5 have a meanvalue of ultimate tensile strength (R_(m)) of 1617 MPa and 1654 MParespectively, measured in the longitudinal direction (Table 3).

TABLE 1 Chemical compositions (wt. %). Steel type C Si Mn P S N Cr Ni ¹A0.2390 0.1720 0.2000 0.0100 0.0018 0.0028 0.3840 0.4760 ¹B 0.2290 0.17900.2000 0.0070 0.0006 0.0024 0.3900 0.5100 ¹C 0.2500 0.1770 0.2000 0.00700.0006 0.0022 0.4000 0.5000 ²D 0.2290 0.1740 1.2000 0.0090 0.0005 0.00230.2100 0.0600 ²E 0.2550 0.1770 1.1900 0.0100 0.0007 0.0026 0.2000 0.0500Steel type Cu Mo Al Nb V Ti B Ca ¹A 0.1600 0.0580 0.0550 0.0010 0.01500.0020 0.0011 0.0013 ¹B 0.1600 0.0500 0.0510 0.0010 0.0100 0.0020 0.00110.0008 ¹C 0.1500 0.0140 0.0580 0.0010 0.0090 0.0020 0.0011 0.0008 ²D0.0100 0.0230 0.0390 0.0010 0.0090 0.0100 0.0015 0.0007 ²E 0.0100 0.03400.0390 0.0010 0.0090 0.0090 0.0013 0.0007 ¹inventive composition²comparative composition

TABLE 2 Manufacturing conditions Steel Strip Hot rolling Cooling Temperannealing strip Steel thickness Heating RT FRT Cooling Cooling AnnealingHolding no. type (mm) temp. (° C.) (° C.) (° C.) temp. (° C.) rate (°C./s) temp. (° C.) time (h) Remarks 1 A 3 1250 1130 890 <100 262 200 8inventive example 2 B 6 1200 1100 900 <100 127 200 8 inventive example 3C 6 1210 1090 890 <100 128 — — inventive example 4 D 3 1270 1140 905<100 290 — — comparative example 5 E 6 1230 1090 905 <100 142 — —comparative example

TABLE 3 Mechanical properties Steel Strip R_(p)0.2 R_(m) A80 strip Steelthickness (L) (L) (L) ChV (−40) (J/cm²) Bending (R_(i)/t) no type (mm)(MPa) (MPa) (%) HBW Longit. Transv. Longit. Transv. Remarks 1 A 3 14131609 4.5 488 105 90 2.6 2.0 inventive example 2 B 6 1310 1511 7.6 467118 85 2.3 1.3 inventive example 3 C 6 1334 1582 7.7 489 78 65 2.8 1.7inventive example 4 D 3 1375 1617 6.3 485 — — 3.7 3.0 comparativeexample 5 E 6 1397 1654 8.0 502 58 50 3.3 2.7 comparative example

1. A hot-rolled steel strip product comprising a composition consistingof, in terms of weight percentages (wt. %): C 0.14-0.35, Si 0-0.5, Mn0.05-0.40, Al 0-0.1, Cu 0.1-0.4, Ni 0.2-0.9, Cr 0.2-0.9, Mo 0-0.2, Nb0-0.005 Ti 0-0.035 V 0-0.05 B 0.0005-0.0050, P 0-0.025, S 0-0.008, N0-0.01, Ca 0-0.01, remainder Fe and inevitable impurities, wherein thesteel product has a Brinell hardness in the range of 420-580 HBW.
 2. Thesteel product according to claim 1, wherein the steel product has aBrinell hardness in the range of 450-550 HBW.
 3. The steel productaccording to claim 1, wherein the steel product has a Charpy-V impacttoughness of at least 50 J/cm² at a temperature of −40° C.
 4. The steelproduct according to claim 1, wherein the steel product has a minimumbending radius of 3.2 t or less in a measurement direction longitudinalto the rolling direction; a minimum bending radius of 2.5 t or less in ameasurement direction transversal to the rolling direction; and whereint is the thickness of the steel strip product.
 5. The steel productaccording to claim 1, wherein the steel product has a microstructureconsisting of, in terms of volume percentages (vol. %), martensite in anamount of at least 90 vol. %; and remainder being retained austenite,bainite, ferrite, pearlite and/or cementite.
 6. The steel productaccording to claim 1, wherein the steel product has a prior austenitegrain size of 50 μm or less.
 7. The steel product according to claim 1,wherein the steel product has a prior austenite grain structure with anaspect ratio in the range of 1.5-7.
 8. The steel product according toclaim 1, wherein the steel strip product has a thickness of 10 mm orless.
 9. A method for manufacturing the steel comprising the followingsteps of providing a steel slab consisting of the chemical compositionaccording to claim 1; heating the steel slab to the austenitizingtemperature of 1150-1300° C.; hot-rolling to the desired thickness at atemperature in the range of Ar₃ to 1250° C., wherein the finish rollingtemperature is in the range of 800° C. to 960° C.; direct quenching thehot-rolled steel strip product to a cooling end and coiling temperatureof 450° C. or less; and optionally, temper annealing at a temperature inthe range of 150° C.-250° C.